For use in such elevator apparatus, a lift drive mechanism is already known which comprises, as shown in FIG. 24, a plurality of ropes 3 reeved around a plurality of sheaves, such as a traction sheave 9 rotatingly driven by a drive motor 91 and direction changing sheaves 4, 43 attached to a cage 1 and a counterweight 2, and having fixed opposite ends 31, 32 for moving the cage 1 and the counterweight 2 upward or downward in opposite directions to each other.
With the elevator apparatus described, the cage 1, counterweight 2 and lift drive mechanism described are arranged in a lift path 10, the cage 1 is guided by guide rails 14, 14 for upward or downward movement, and the counterweight 2 is guided by guide rails 15, 15 for upward or downward movement as shown in FIG. 25. The drive motor 91 has connected thereto an unillustrated control circuit for controlling the upward and downward movement of the cage 1 and stopping of a cage door 11 at a position coinciding with a floor door 12.
With the conventional elevator apparatus shown in FIGS. 24 and 25, the rope 3 reeved around the traction sheave 9 must move with the rotation of the sheave 9 without slipping relative to the sheave 9 while the traction sheave 9 is being rotated by the drive motor 91 to move the cage 1 upward or downward. For this reason, the prior art has encountered problems such as difficulty in reducing the weight of the cage 1.
To render the rope 3 of the conventional elevator apparatus reeved around the traction sheave 9 free of slipping in the case where the rope 3 is subjected to tension T1 on the slack side thereof and to tension T2 on the tensioned side thereof as shown in FIG. 26, the relationship of Mathematical Expression 1 (Eytelwein) needs to be satisfied, assuming that the coefficient of friction between the traction sheave 9 and the rope 3 is μ and that the angle of the rope 3 reeved around the sheave 9 is θ.
(Mathematical Expression 1)T2/T1≦exp(μ·θ)
Suppose the tension T1 on the slack side is due to the weight of the cage 1. When a small number of passengers are in the cage 1, the tension T1 is small, and the rope 3 is likely to slip, failing to satisfy the relationship of Mathematical Expression 1. For example, suppose the cage 1 itself has weight of 1500 Kg, the loading capacity of the cage is 1000 Kg, and the weight of the counterweight 2 is the weight of the cage 1 plus 50% of the loading capacity. The left side member of Expression 1 has the following values when the weight of load is zero and when the cage is fully loaded.
(Mathematical Expressions 2)T2/T1=2000/1500=1.33T2/T1=2500/2000=1.25
If the weight of the cage 1 itself is then reduced to 1000 Kg, the values of Mathematical Expressions 2 are as follows.
(Mathematical Expressions 3)T2/T1=1500/1000=1.5T2/T1=2000/1500=1.33
Thus, a change in the weight of the cage itself or in the weight of load greatly varies the value of the left side member (T2/T1) of Expression 1 to be satisfied. This value increases especially with a reduction in the weight of the cage 1, giving rise to the problem that the cage 1 cannot be reduced in weight.
It is therefore conventional practice to attach a weight to the cage 1 so as not to permit the rope 3 to slip even when the cage carries a small number of passengers. This gives increased weight to the cage 1 itself. The increase in the weight of the cage 1 itself gives rise to the problem of making the lift drive mechanism large-sized and heavier. Furthermore, the drive motor 91 serving as the power source for the lift drive mechanism is given an increased capacity, consequently resulting in increased power consumption and also entailing the problem of necessitating space for the installation of the drive motor 91 which becomes greater in size.
An object of the present invention is to overcome all the foregoing problems by providing a drive mechanism capable of reciprocatingly driving a cage or like movable body without using any traction sheave and an elevator apparatus of the novel reciprocating drive type having the drive mechanism incorporated therein.